Hitherto, various types of seat belt retractors including pretensioners have been developed for seat belt apparatuses to be provided in vehicles such as automobiles. In a pretensioner, in early stages of emergency such as a collision of a vehicle when a vehicle has undergone a deceleration greater than a normal deceleration, a spool of a seat belt retractor is rotated in a seat-belt-retracting direction with the use of a reactant gas generated by a gas generator, whereby the seat belt is retracted around the spool. Thus, the slack in the seat belt is quickly eliminated and a tension is applied to the seat belt, whereby the force of restraining the occupant is increased.
Examples of such a conventional pretensioner include a pretensioner in which a plurality of balls as force transmitting members are provided in a pipe (see PTL 1, for example). In times of emergency, the balls move within and along the pipe under a gas pressure produced by a reactant gas generated by a gas generator. The balls are guided along a guide surface of a guide member screwed to the inside of an open end portion of the pipe that is on a side opposite the side having the gas generator. The balls come out of the pipe via a cut portion provided in the pipe and push a plurality of pushing portions provided on a ring gear. The ring gear thus pushed rotates and causes a spool to rotate in a seat-belt-retracting direction, whereby the seat belt is retracted.